I have fairly recently started reloading for my Martini Henry Mk IV B and brought a 12mm diameter card wad punch (the cast iron type that you whack with a mallet) to make the over-powder wads with. My case inside neck diameter is 11.91mm / 0.469", or there about's, so thought the 12mm wad punch would be the one to go for. What I wasnt expecting was the punch to be 1/2" diameter and the wads to be a right pain in the derrière to get started and seated properly. The manufacturer then stated that the 12mm and 1/2" punches are the same

and doing a quick web search this same scenario is repeated with other makers.
I made up a 10mm stainless rod with a 2mm chamfer one the end and now press the card wads into a 1/4" piece of silicone sheet and they turn out looking a bit like a cup cake case with a nice uniform crinkled edge that gives and moves to whatever variance I get with the case neck. They grip nicely as well. Can anyone see anything wrong with doing this?
Just as a footnote, the last batch of ammunition I made up with these wads seemed much more accurate than my previous efforts but it could have been down to a number of other things as well.